1. Field of the Invention
The present invention relates to printing apparatuses that detect a position of a carriage using an encoder sensor.
2. Description of the Related Art
Conventionally, inkjet printing apparatuses are known that carry out image printing onto a printing medium by discharging ink from nozzles of a printhead. Generally, these inkjet printing apparatuses are provided with a carriage on which a printhead and an ink tank are mounted, a conveyance mechanism that conveys the printing medium, and a control mechanism that controls operations of these. The carriage, on which is mounted the printhead from which ink droplets are discharged from multiple nozzles, is caused to scan in a direction (main scanning direction) that is orthogonal to a conveyance direction (sub scanning direction) of printing papers. The inkjet printing apparatus is able to carry out printing of an entire image region by performing multiple scan and conveyance operations of the printing medium in which ink is discharged onto the printing medium during each scan while on the other hand the printing medium is intermittently conveyed between each scan. In a case of carrying out color image printing, this is achieved by overlaying ink droplets discharged from multiple printheads corresponding to multiple types of ink colors, or by causing the ink droplets to land adjacent to each other.
When the carriage is caused to scan, encoder signals, which are constituted by an A phase signal and a B phase signal as shown in FIGS. 12A and 12B, are outputted from an optical or magnetic rotary encoder or linear encoder provided on the carriage. Here, it is usual for the A phase signal and the B phase signal to have a phase difference of 90 degrees from each other. The inkjet printing apparatus is able to specify the scanning direction (forward direction or return direction) and the position of the carriage according to these encoder signals. In Japanese Patent Laid-Open Nos. 07-205485 and 07-205487, description is given of an encoder counter referencing the above-mentioned encoder signals and increasing or decreasing a counter value in response to the scanning direction of the carriage, thereby obtaining a relative distance from a predetermined carriage reference position.
In recent years, accompanying higher resolutions and a greater number of nozzles in the printhead of inkjet printing apparatuses, the scale of the control circuits of printhead is also increasing. Accompanying higher resolutions and a greater number of nozzles in the printhead, it is common for the control circuits of the printhead to be configured as shown in FIG. 13. That is, as shown in FIG. 13, the printhead is divided into multiple control units in the manner of control unit 1, control unit 2, and so on up to a control unit n, and control is performed in regard to each printhead control unit by a printhead control IC 1, printhead control IC 2, printhead control IC 3, and so on up to a printhead control IC n corresponding to the printhead control units referencing the encoder signals from the encoder.
Configurations such as that shown in FIG. 13 are commonly used since they have the merit of enabling shorter development times and reducing costs due to the configuration of each of the ICs being simpler compared to a case where control of the printhead control units is performed from a single control circuit. Furthermore, there is also the merit that configurations can be achieved flexibly using multiple printhead control ICs in cases where the printhead control units vary according to the model of inkjet printing apparatus.
There are various methods available for resetting the encoder counter inside each of the printhead control ICs to set the carriage reference position with the configuration shown in FIG. 13. For example, as shown in FIG. 14, there is a method by which a register of the encoder counter inside each of the printhead control ICs is reset by the CPU via a bus. Furthermore, as shown in FIG. 15, there is a method in which a common encoder counter reset signal is connected to each of the printhead control ICs, and the register of the encoder counter inside each of the printhead control ICs is reset according to the encoder counter reset signal after causing the carriage to stop.
In the configurations shown in FIG. 14 and FIG. 15, it is extremely difficult to reset the encoder counter register in each of the printhead control ICs using a same timing. This is because in the configuration shown in FIG. 14, the CPU carries out sequential reset operations for the printhead control ICs, and therefore it is not possible to perform the reset operation for all the printhead control ICs using the same timing. In the configuration shown in FIG. 15, the printhead control ICs can undergo the reset operation in common according to the encoder counter reset signal, but due to internal skew differences originating in manufacturing discrepancies of each of the printhead control ICs, the timing for the resets cannot be made simultaneous.
Chattering sometimes occurs in the encoder signal according to the positional relationship between the carriage and the encoder slits. That is, in a case where the carriage is in such a position, the encoder signal becomes unstable. In the configurations shown in FIG. 14 and FIG. 15, suppose for example that a reset operation is carried out in a printhead control IC having a CPU. And if there is undesirable oscillation in the encoder signal when a reset operation is to be carried out in a different printhead control IC at a timing shifted from that time point, then the count value advances undesirably in the printhead control IC for which the reset operation was carried out initially. That is, unfortunately the carriage reference positions become set differently between the printhead control ICs. When the settings of the reference positions between each of the printhead control ICs become undesirably different, the registration shifts between the printhead control units controlled by each of the printhead control ICs, which incurs a deterioration in image quality.